This invention relates to electronic control systems for electric motors and, more particularly to a servo system for a self-synchronous brushless motor.
DC brushless motors are often used in servo control systems in a wide variety of applications. Typically, brushless motors are constructed with stationary windings and a rotary permanent magnet field. The windings are commutated by solid state switches controlled in response to signals from rotor position sensors. Since the brushes and commutator of the normal DC motor are eliminated, and since commutation is controlled by solid state switches, the brushless motors are well suited for electronic servo system control.
In such a servo system, for example a torque control system, it is desirable to maintain a linear relationship for both positive and negative values of servo loop error under conditions of both positive and negative values of the command signal. The foregoing conditions represent four separate states, sometimes referred to as four-quadrant control. The positive and negative command signals correspond to desired clockwise and counterclockwise torque during either clockwise or counterclockwise rotation of the motor. Two of these states correspond to normal motoring modes wherein both the command signals and the error signals have the same sense, either positive or negative. In the motoring modes the motor is driven from the power source. The other two states correspond to regenerative modes wherein the command signal and the error signal have an opposite sense, one being positive while the other is negative. In the regenerative mode, the internally generated motor current is utilized to produce the desired torque.
An object of this invention is to provide a four-quadrant control system for a brushless motor which is capable of making a smooth automatic transition from one quadrant to another while maintaining a linear control relationship.
Another object is to provide a servo system for a brushless motor capable of achieving efficient control through relatively simple reliable control electronics.